With PROCESS_SEPARATE_RULE in wafsamba it is now possible to simplify
configuration and checks for MIT/Heimdal Kerberos implementations.

1. Move MIT krb5 checks from source3/wscript to wscript_configure_krb5
2. Make sure they are called same way (--with-mit-krb5-checks)
3. If no configure checks identified MIT krb5 in system (or were disabled),
make sure Heimdal build is selected, embedded (default) or system-provided.

This makes logic of configuration unchanged for Heimdal builds but adds
less hacky way to use MIT krb5 builds. The latter does not work yet as we
need to untangle more subsystems from HDB/Heimdal-specific details but
lays out a foundation for that.